Abstract: The Oncemelania is middle parasit izeder of Schistosome Controlling it is the important way to prevent the Schistosomula.The article mainly introduces the measurement of jet flow field's velocity in pressure conduit by FVM numerical modeling and the high-tech particle image velocity, and analyses the reason why the Oncemelania can sink to the bottom but not escape from the equipment named Oncemelania-free intake devices.